A variety of techniques in molecular biology and molecular medicine now rely on analysis of single biological molecules. Such techniques include DNA and RNA sequencing, polymorphism detection, the detection of proteins of interest, the detection of protein-nucleic acid complexes, and many others. The high sensitivity, high throughput and low reagent costs involved in single molecule analysis make this type of analysis an increasingly attractive approach for a variety of detection and analysis problems in molecular medicine, from low cost genomics to high sensitivity marker analysis.
For example, single molecule DNA sequencing is useful for the analysis of large sets of related DNAs, such as those that occur in a genome. In certain of these methods, a polymerase reaction is isolated within an array of extremely small (typically optically confined) observation volumes that each permit observation of the enzymatic action of individual polymerases in each reaction/observation volume of the array, while the polymerase copies a template nucleic acid. Nucleotide incorporation events are individually detected, ultimately providing the sequence of the template molecule. This approach dramatically increases throughput of sequencing systems, and also dramatically reduces reagent consumption costs—to the point where personalized genomics is increasingly feasible.
The small observation volumes used for single molecule nucleic acid sequencing and other analysis methods are typically provided by immobilizing or otherwise localizing the polymerase (or other) enzyme within an optical confinement reaction/observation region, such as an array of extremely smalls wells as in an array of Zero Mode Waveguides (ZMWs), and delivering a template, primers, etc., to the reaction region. For a description of ZMW arrays and their application to single molecule analyses, and particularly to nucleic acid sequencing, see, e.g., “Selective aluminum passivation for targeted immobilization of single DNA polymerase molecules in zero-mode waveguide nanostructures” (2008) Korlach et al. Proceedings of the National Academy of Sciences U.S.A. 105(4): 1176-1181; “Improved fabrication of zero-mode waveguides for single-molecule detection” (2008) Foquet et al. Journal of Applied Physics 103, 034301; “Zero-Mode Waveguides for Single-Molecule Analysis at High Concentrations” Levene et al. Science 299:682-686; published U.S. patent application No. 2003/0044781; Eid et al. (2008) “Real-Time DNA Sequencing from Single Polymerase Molecules” Science DOI: 10.1126/science.322.5905.1263b; and U.S. Pat. No. 6,917,726, each of which is incorporated herein by reference in its entirety for all purposes.
One difficulty in performing single molecule analyses occurs in loading the reaction/observation region of single molecule analysis devices with the molecules of interest (e.g., template or other analyte and/or enzyme). Loading two or more molecules of interest into a ZMW or other small observation volume tends to complicate any analysis of signals observed from double (or more than double)-loaded region. This is because two (or more) sets of signals may simultaneously be observed from the ZMW or other observation volume, meaning that the signals from the ZMW would have to be deconvoluted before data from the observation region could be used. More typically, data from double (+) loaded ZMWs can be recognized by various data analysis methods, and data from mis-loaded ZMWs or other relevant observation volumes is simply discarded.
To reduce the incidence of multiple molecule loading events in the relevant reaction/observation volume(s) of the array, it is typical in the art to substantially “under-load” the array with the analyte molecules of interest. Random distribution of molecules into the array results in one or fewer molecules being loaded into most reaction/observation volumes when fewer than 37% of all observation volumes are loaded. This type of loading is referred to as “Poisson-limited” analyte loading, meaning that few enough molecules are added to the array so that a Poisson-style random statistical distribution of the analytes into the array results in one or fewer analytes per observation volume in most cases. In the ZMW context, state of the art yields for single-molecule occupancies of approximately 30% have been obtained for a range of ZMW diameters (e.g., 70-100 nm). See, Foquet (2008), above. For this degree of loading, about 60% of the ZMWs in a typical ZMW array are not loaded (e.g., have no analyte molecules).
While such random distribution methods are effective in ensuring that, in most cases, not more than a single template or enzyme (or other analyte) molecule is loaded in each observation/reaction volume in an array such as a ZMW array, it would be desirable to develop methods and compositions for increasing the template and enzyme loading density of such arrays. Higher loading densities would permit the simultaneous analysis of more analyte molecules in the array, increasing the throughput of such systems, while simultaneously decreasing analysis costs. The present invention provides these and other features that will be apparent upon complete review of the following.